Fix Gutenberg plugin render registration

[akirk]

During the recent Gutenberg release (see this Slack thread) we discovered that the Public Post Preview was missing on the editor in https://make.wordpress.org/core/ which runs the latest Gutenberg version.

I then discovered that since Gutenberg v23.4.0 the composed PreviewToggle component is rejected when it is passed directly as the render value to registerPlugin.

This was introduced in WordPress/gutenberg#78674. Because it changed withSelect from returning a pure(...) wrapped component to returning a memo(...) wrapped component, this error was thrown:

The "render" property must be specified and must be a valid function. <anonymous code>:1:147461
    overrideMethod <anonymous code>:1
    ra index.ts:157
    <anonymous> gutenberg-integration.js:2
    <anonymous> gutenberg-integration.js:2

Because this plugin's PreviewToggle export is composed with withSelect as the outermost higher-order component, it became a React memo object rather than a plain function. Simply wrapping the component in an anonymous function fixes that.

This change maintains compatibility with older WordPress/Gutenberg versions. Because the registered render value is now a plain function, it passes the old typeof render === 'function' validation, and Gutenberg's PluginArea still renders it as a normal component.

[akirk]

We might equally well close this, as I realize that this will be fixed in Gutenberg 23.5.0 (WordPress/gutenberg#79315) during which release, ironically, this issue was discovered.